Light-emitting device comprising a gallium-nitride-group compound-semiconductor

ABSTRACT

In the light-emitting gallium-nitride-group compound semiconductor devices using a substrate, the operating voltage is lowered and at the same time the occurrence of crack during crystal growth is suppressed, resulting in an improved manufacturing yield rate. The device includes a stacked structure of an n-type layer, a light-emitting layer and a p-type layer formed in the foregoing order on a substrate, and an n-side electrode formed on the surface of the n-type layer. The n-type layer is a laminate layer composed of, in the order from the substrate, first n-type layer and a second n-type layer having a carrier concentration higher than that of the first n-type layer. As the contact resistance between the n-type layer and the n-side electrode formed thereon is reduced, the operating voltage of a light-emitting device is lowered, and the power consumption decreased.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a light-emitting device madewith gallium-nitride-group compound-semiconductor such as light-emittingdiode, laser diode, etc.

[0002] Gallium-nitride-group compound-semiconductors have beenincreasingly used as the semiconductor material for the visiblelight-emitting devices and for use in the electronic devices of highoperating temperature. The development has been significant in the fieldof blue and green light-emitting diodes.

[0003] In manufacturing the gallium-nitride-groupcompound-semiconductors devices, an insulating sapphire is generallyused as the substrate for growing semiconductor film. Such devices aredifferent from the light-emitting devices where semiconductor substratesother than gallium-nitride-group type substrates, such as, for exampleGaAs or InGaP, are utilized. Specifically, those using an insulatingsubstrate like the present sapphire have the n-side and p-sideelectrodes formed in one side of the substrate wherein the semiconductorfilm has been formed, because the electrodes can not be provided fromthe substrate.

[0004] Meanwhile, in the recent manufacture of light-emitting devices,including those using the sapphire substrate, the growing ofgallium-nitride-group semiconductor thin film by a metal organic CVDmethod has become a main stream procedure. In the procedure, a substrateis placed in a reaction tube, and metal organic compound gas(trimethyl-gallium [TMG], tri-methyl-aluminum [TMA], tri-methyl-indium[TMI], etc.) are supplied therein as the material gas for the Group IIIelement, and ammonia, hydrazine, etc. as the material gas for the GroupV element, while maintaining the substrate at a high temperature 900°C.-1100° C., to have an n-type layer, a light-emitting layer and ap-type layer grown on the substrate in a stacked structure. After thelayers are grown and formed, the p-type layer and the light-emittinglayer are partially etched off to have the n-type layer exposed, andthen an n-side electrode and a p-side electrode are formed on thesurface of exposed n-type layer and the p-type layer, respectively, forexample by a deposition method.

[0005] Most of the recent light-emitting devices have the abovedescribed double-hetero-structure, fabricated by stacking the thin filmsof gallium-nitride-group compound-semiconductor on a sapphire substrate.FIG. 2 shows a cross sectional structure of a prior art light-emittingdevice of gallium-nitride-group compound-semiconductor.

[0006] In FIG. 2, a buffer layer 12, an n-type layer 13 ofgallium-nitride (GaN), a light-emitting layer 14 ofindium-gallium-nitride (InGaN), a p-type clad layer 15 ofaluminum-gallium-nitride (AlGaN) and a p-type contact layer 16 of GaNare stacked on a sapphire substrate 11. A p-side electrode 17 is formedon the p-type contact layer 16, and an n-side electrode 18 is formed onan exposed surface of the n-type layer 13 provided by partially removingthe following three layers, p-type contact layer 16, p-type clad layer15 and light-emitting layer 14. The n-type electrode 18 is normally madewith aluminum (Al), titanium (Ti), gold (Au), or such other metals. Thelight-emitting gallium-nitride-group compound-semiconductor devices ofthe above structure have been disclosed in, for example, Japanese PatentPublication No. 6-268259.

[0007] In the prior art light-emitting gallium-nitride-groupcompound-semiconductor devices of the above structure, the n-type layer13 is formed of a gallium-nitride-group compound-semiconductor dopedwith n-type impurities such as silicon (Si), germanium (Ge). Morespecifically, during the growth of the n-type layer ofgallium-nitride-group compound-semiconductor by said metal organic CVDmethod, silane, mono-methyl-silane, etc. are supplied, together with thematerial gas, as material gas for Si, or germane, mono-methyl-germane,etc. as material gas for Ge. The carrier concentration of n-type layer13 may be controlled by adjusting the flow rate of the material gas forn-type impurities.

[0008] In the gallium-nitride-group compound-semiconductor, the n-typelayer may also be formed by intentionally not doping the n-typeimpurities, because it exhibits the n-type property even without then-type impurities being doped therein.

[0009] If in the light-emitting gallium-nitride-groupcompound-semiconductor devices the efficiency of light-emission is to bemaintained high, the operating voltage needs to be lowered. In order toreduce the operating voltage, the series resistance in respective layersof compound-semiconductor stacked on the substrate 11 and the contactresistance with electrode have to be made low.

[0010] An effective means for reducing the series resistance of n-typelayer 13 and the contact resistance with the n-side electrode 18 is toincrease the doping quantity of n-type impurities during growth ofn-type layer 13 by metal organic CVD. However, when doping quantity ofthe n-type impurities is increased, a strain can be generated in thegrown n-type layer 13, which increases and readily leads to cracks atthe n-type layer 13. If there are cracks in the n-type layer 13, an evenemission of light may not be obtained over the entire surface, and thereliability of a light-emitting device may be degraded.

[0011] On the other hand, if priority is placed on suppression of cracksat n-type layer 13, the n-type layer 13 needs to be grown and formed ina reduced doping quantity of the n-type impurities. In this case,however, it becomes difficult to reduce the contact resistance withn-side electrode 18. If, in compensation of the above, the layerthickness of the n-type layer 13 is increased up-to about several μm(e.g. 16, 17 μm) in order to reduce the series resistance of alight-emitting device, cracks are easily induced like in the earlierdescribed case. In addition, it needs a longer time for growing thecrystal, which is an additional disadvantage in the manufacture thereof.

[0012] As described in the above, if in a light-emittinggallium-nitride-group compound-semiconductor device the doping quantityof n-type impurities is increased for lowering the operating voltage, orthe layer thickness is increased, the occurrence of cracks may beunavoidable, which leads to a degraded light-emitting capability and adeteriorated manufacturing yield rate.

[0013] The problems expected to be solved by the present invention witha light-emitting gallium-nitride-group compound-semiconductor deviceusing an insulating substrate are; first to reduce the operatingvoltage, and second to suppress the occurrence of cracks during growthfor an improved manufacturing yield rate.

SUMMARY OF THE INVENTION

[0014] A novel invented light-emitting gallium-nitride-groupcompound-semiconductor device of the present invention has a stackedstructure comprising an n-type layer, a light-emitting layer and ap-type layer formed one after the other on an insulating substrate. Inaccordance with the present invention, the p-type layer andlight-emitting layer are partially removed from the surface of thestacked structure formed on the substrate such that the n-type layer isexposed, and then an electrode is formed on the exposed surface of then-type layer. The n-type layer contains, in order from the substrate, atleast a first n-type layer and a second n-type layer whose carrierconcentration is higher than that of the first n-type layer. Theelectrode is disposed on the second n-type layer.

[0015] With the above structure, a light-emitting gallium-nitride-groupcompound-semiconductor device is formed, in which the operating voltageis low and the manufacturing yield rate is high.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a cross sectional view shows the structure of alight-emitting gallium-nitride-group compound-semiconductor device inaccordance with an exemplary embodiment of the present invention.

[0017]FIG. 2 is a cross sectional view shows one of the structures of adevice of a light-emitting gallium-nitride-group compound-semiconductorin a prior art.

[0018]FIG. 3 is a sectional view showing a structure of semiconductorlight emitting device of gallium nitride compound in another embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] A first exemplary embodiment of the light-emittinggallium-nitride-group compound semiconductor device of the presentinvention has a stacked structure comprising an n-type layer, alight-emitting layer and a p-type layer formed one after the other on aninsulating substrate. The p-type layer and light emitting layer arepartially removed from the surface of the stacked structure formed onthe substrate to expose the n-type layer, and an electrode is formed onthe exposed surface of n-type layer. The n-type layer comprises, in theorder from the substrate, at least a first n-type layer and a secondn-type layer whose carrier concentration is higher than that of thefirst n-type layer. The electrode is disposed on the second n-typelayer. As a result of the present invention, the operating voltage ofthe light-emitting device is lowered through a reduced contactresistance between the n-type layer and the n-side electrode.

[0020] In a second embodiment of the light-emitting device of thepresent invention the carrier concentration of the first n-type layer isset within a range 1×10¹⁶-2×10¹⁸ cm⁻³, and the carrier concentration ofthe second n-type layer within a range 2×10¹⁸-1×10¹⁹ cm⁻³. As a result,the occurrence of cracks is suppressed and the contact resistancebetween the n-type layer and the electrode is reduced.

[0021] In a third embodiment of the light-emitting device of the presentinvention the thickness of the second n-type layer is smaller than thatof the first n-type layer. As such, the occurrence of cracks issuppressed.

[0022] In a fourth embodiment of the light-emitting device of thepresent invention the layer thickness of the first n-type layer is setwithin a range of 1-5 μm, the thickness of the second n-type layer isset within a range of 0.1-0.5 μm. By specifying a range in the thicknessof n-type layer, the occurrence of cracks is effectively suppressed, andthe depth of etching can be maintained with a high precision whenremoving a part of the p-type layer and the light-emitting layer forexposing the surface of the n-type layer.

[0023] Next, the exemplary embodiments of the present invention aredescribed with reference to the drawing.

[0024]FIG. 1 is a cross sectional view used to show the structure of alight-emitting gallium-nitride-group compound-semiconductor device inaccordance with an exemplary embodiment of the present invention.

[0025] In FIG. 1, a buffer layer 2 is formed on a substrate 1 made ofsapphire. The buffer layer 2 may be formed of GaN, GaAlN, AlN, AlInN,etc. Stacked on the buffer layer 2 are, in the order from the bottom, ann-type layer 3, a light-emitting layer 4, a p-type clad layer 5 and ap-type contact layer 6. The n-type layer 3 has a laminate structurecomprising, in the order from the substrate 1, a first n-type layer 31of lower carrier concentration and a second n-type layer 32 of highercarrier concentration. Si, Ge, etc. may be used as n-type impurities forforming the n-type layer 3.

[0026] The light-emitting layer 4 of InGaN may be formed either bydoping zinc, Si, etc. for making use of the impurity level; or leavingit as the un-doped, keeping thinner than 10 nm, for making use of thequantum level. “Un-doped” means the one in which neither p-typeimpurities nor n-type impurities are added during the growth process.

[0027] The p-type clad layer 5 can be formed of AlGaN, GaN, AlGaInN,etc. The p-type contact layer 6 may be formed of GaN, InGaN, etc. Thep-type impurities to be doped in the p-type clad layer 5 and the p-typecontact layer 6 are Mg.

[0028] A p-side electrode 7 is provided on the p-type contact layer 6,and an n-side electrode 8 is provided on the second n-type layer 32among the n-type layers 3. The n-side electrode 8 may be formed ofaluminum(Al), titanium(Ti), etc.

[0029] As described earlier, if in the manufacture of a light-emittinggallium-nitride-group compound-semiconductor device the doping quantityof n-type impurities in the n-type layer is increased or the thicknessof n-type layer is increased for the purpose of lowering the operatingvoltage, the frequency of the generation of cracks during growingprocess increases, and the manufacturing yield rate decreases. One ofthe reasons for the generation of cracks is believed to be attributableto a strain created in the n-type layer grown by doping n-typeimpurities with high concentration, and the amount of strain increasesalong with the increasing thickness of the layer.

[0030] Namely, in a prior art technology there was a tradeoff betweenthe amount of doping of the n-type impurities and the thickness ofn-type layer regarding the occurrence of cracks. When attempting tolower the operating voltage through optimization of the amount of n-typeimpurities to be doped in the n-type layer and the thickness of thelayer. Hence, there was a limitation in the conventional technology. Inother words, because there is a tradeoff in the relationship between thedoping amount of n-type impurities in the n-type layer and the layerthickness with respect to occurrence of cracks in n-type layer,attempting to lower the operating voltage through the optimization ofthe foregoing factors has a limitation.

[0031] In the present embodiment where the n-type layer 3 has a laminatestructure comprising, in the order from the substrate 1, a first n-typelayer 31 of lower carrier concentration and a second n-type layer 32 ofhigher carrier concentration, and the n-side electrode 8 is disposed onthe second n-type layer 32, the operating voltage can be lowered whilekeeping the manufacturing yield rate high. Detailed description is madein the following.

[0032] A light-emitting gallium-nitride-group compound-semiconductordevice of the present exemplary embodiment is shown in FIG. 1. Asstated, the device comprises an n-type layer 3 having a laminatestructure comprising, in the order from the substrate 1, a first n-typelayer 31 of lower carrier concentration and a second n-type layer 32 ofhigher carrier concentration. Namely, the first n-type layer 31 of lowercarrier concentration is formed on the substrate 1, and then the secondn-type layer 32 of higher carrier concentration is formed thereon. Then-side electrode 8 is formed on the second n-type layer 32.

[0033] In the above laminate structure where the first n-type layer 31and the second n-type layer 32, the carrier concentration of respectivelayers being different, are formed one on the other in the order fromthe substrate 1, the first n-type layer 31 can be formed thick with thedoping quantity of n-type impurities in the first n-type layer 31decreased for a low carrier concentration. Therefore, both the increaseof resistance in the first n-type layer and the occurrence of cracks issuppressed. By disposing the n-side electrode 8 on the second n-typelayer 32 which has been formed on the first n-type layer 31 with then-type impurities doped more than in the first n-type layer 31 for ahigher carrier concentration, the contact resistance between the secondn-type layer 32 and the n-side electrode 8 is reduced. Therefore, theoperating voltage of a light-emitting device is lowered and the powerconsumption reduced.

[0034] As described in the above, each of the respective first andsecond n-type layers 31, 32 have different carrier concentrations. Thisenables the optimization in both the lowering of operating voltage andthe prevention of cracks. According to the results of experiments,preferred values of carrier concentration have been determined to be asfollows.

[0035] It is preferred that the carrier concentration of the firstn-type layer 31 be within a range of 1×10¹⁶ cm⁻³-2×10 ¹⁸ cm⁻³. When thecarrier concentration of first n-type layer 31 is lower than 1×10¹⁶ cm⁻³the series resistance of the first n-type layer 31 itself goes high andthe operating voltage of the device tends to go high; on the other hand,when the carrier concentration of first n-type layer 31 is higher than2×10¹⁸ cm³ it tends to invite the cracks.

[0036] It is preferred that the carrier concentration of the secondn-type layer 32, whose carrier concentration is higher than that of thefirst n-type layer 31, is in the range of 2×10¹⁸ cm⁻³-1×10¹⁹ cm⁻³. Whenthe carrier concentration of second n-type layer 32 is smaller than2×10¹⁸ cm⁻³ the contact resistance with the n-side electrode 8 can notbe reduced low enough. On the other hand, when the carrier concentrationof second n-type layer 32 is greater than 1×10¹⁹ cm⁻³ the crystallinityof the layer tends to be degraded. This may deteriorate thecrystallinity of light-emitting layer and a p-type layer to be grownthereon, and the light-emitting output can be degraded.

[0037] Further, it is preferred that the thickness of second n-typelayer 32 has a thickness smaller than that of the first n-type layer 31.Specifically, the thickness of second n-type layer should preferably bewithin a range of 0.1-0.5 μm. If it is thinner than 0.1 μm, it becomesdifficult to control the etching depth in the process of partiallyremoving the p-type layer comprising the p-type clad layer 5 and p-typecontact layer 6 as well as the light-emitting layer 4 for having thesurface of n-type layer 3 exposed. If it is thicker than 0.5 μm, thecrystallinity of second n-type layer 32 is degraded, and thecrystallinity of a light-emitting layer 4, a p-type clad layer 5 and ap-type contact layer 6 to be grown on the second n-type layer 32 arealso degraded. Thus, the light-emitting output can be degraded.

[0038] The thickness of first n-type layer 31 should preferably bewithin a range 1-5 μm. If it is thinner than 1 μm, the series resistanceof the device goes high to a raised operating voltage. If the firstn-type layer is thicker than 5 μm, the cracks may readily occur.

[0039] (Embodiment)

[0040] An exemplary method for manufacturing an invented light-emittingsemiconductor device is described below.

[0041] The present exemplary embodiment refers to a method of growing agallium-nitride-group compound-semiconductor using a metal organic CVDprocess.

[0042] (Embodiment 1)

[0043] Description is made with reference to FIG. 1.

[0044] A sapphire substrate 1 having a mirror-polished surface is set ona substrate holder within a reaction tube. The substrate 1 is maintainedat 1100° C. on the surface for 10 minutes, and is cleaned to removeorganic stains or humidity sticking on the surface by heating thesubstrate in a hydrogen gas flow.

[0045] Then, the surface temperature of substrate 1 is lowered down to600° C., and a buffer layer 2 of AlN is grown to the thickness ofapproximately 25 nm by providing nitrogen gas, as the main carrier gas,at 10 liter/min., ammonia at 5 liter/min., a TMA (tri-methyl-aluminum)carrier gas containing tri-methyl-aluminum at 20 cc/min.

[0046] The supply of the TMA carrier gas is discontinued and thetemperature is raised up-to 1050° C. Then, while continuing the flow ofnitrogen gas, as the main carrier gas, at 9 liter/min., and hydrogen gasat 0.95 liter/min., new gases are added, namely, a carrier gas oftri-methyl-gallium (TMG) at 4 cc/min., a 10 ppm SiH4 (mono-silane) gas,as the source of Si, at 10 cc/min. for a duration of 60 min. in order togrow a first n-type layer 31 of Si doped GaN in the thickness ofapproximately 2 μm. As the result, the carrier concentration of firstn-type layer 31 is 1×10¹⁸ cm⁻³.

[0047] After the first n-type layer 31 is grown and formed, whilekeeping the respective flow rates of the main carrier gas and the TMGcarrier gas as they are, the flow rate of SiH4 gas alone is modified to50 cc/min. to be continued for 6 min. in order to grow a second n-typelayer 32 of Si doped GaN in the thickness of 0.2 μm. As the result, thecarrier concentration of the second n-type layer 32 is 5×10¹⁸ cm⁻³.

[0048] After the second n-type layer 32 is grown and formed, flow of theTMG carrier gas and the SiH4 gas are discontinued, the surfacetemperature of substrate 1 is lowered down to 750° C., and new gases areprovided; nitrogen gas, as the main carrier gas, at 10 liter/min., a TMGcarrier gas at 2 cc/min., and a TMI (tri-methyl-indium) carrier gas at200 cc/min. for a duration of 30 sec. in order to grow a light-emittinglayer 4 of un-doped InGaN in the thickness of approximately 3 nm.

[0049] After the light-emitting layer 4 is formed, the TMI carrier gasand the TMG carrier gas are discontinued, the surface temperature ofsubstrate 1 is raised up-to 1050° C., and new gases are provided;nitrogen gas, as the main carrier gas, at 9 liter/min., hydrogen gas at0.94 liter/min., a TMG carrier gas at 4 cc/min., a TMA carrier gas at 6cc/min., a carrier gas for bis-cyclo-pentadienyel-magnesium (Cp₂Mg), orMg source, at 50 cc/min. for a duration of 4 min. in order to grow ap-type clad layer 5 of Mg doped AlGaN in the thickness of approximately0.1 μm.

[0050] Then, the TMA carrier gas alone is discontinued, and at 1050° C.,new gases are provided, namely, nitrogen gas, as the main carrier gas,at 9 liter/min., hydrogen gas at 0.90 liter/min., a TMG carrier gas at 4cc/min., a Cp₂Mg carrier gas at 10 cc/min., for a duration of 3 min. inorder to grow a p-type contact layer 6 of Mg doped GaN in the thicknessof approximately 0.1 μm. After the p-type contact layer 6 is grown, theTMG carrier gas, as a material gas, and ammonia are discontinued. Thewafer is then cooled down to room temperature while the flow of nitrogengas and the hydrogen gas are maintained as is, and then the wafer istaken out from the reaction tube.

[0051] On the surface of the stacked layer structure containing aquantum well structure thus formed of gallium-nitride-group compoundsemiconductor, an SiO₂ film is deposited by CVD process. Then, anetching mask is patterned in a certain specific shape byphotolithography. The p-type contact layer 6, the p-type clad layer 5and the light-emitting layer 4 are etched off in part by a reactive ionetching process for a depth of approximately 0.25 μm to have the surfaceof second n-type layer 32 exposed. On the exposed surface of secondn-type layer 32 an n-side electrode 8 of Al is deposited and formedthrough photolithography and deposition processes. By the sameprocedure, a p-side electrode 7 of Ni and Au is deposited and formed onthe surface of the p-type contact layer 6.

[0052] The reverse surface of the sapphire substrate 1 is polished downto approximately 100 μm thick, and separated into chips by scribing.Each of the chips is attached on a stem with the surface having theelectrode formed thereon. The respective n-side electrode 8 and p-sideelectrode 7 on the chip are connected with wire to electrodes of thestem, and the whole structure is resin-molded to complete alight-emitting diode. When the light emitting diode is driven with 20 mAforward current, it emits a blue-violet light of 430 nm wave-length; theforward operating voltage at that time was 3.4V.

[0053] As a comparative specimen 1, a light-emitting diode has beenmanufactured in the same procedure as in embodiment 1, except that; inthe process for growing a first n-type layer 31 and a second n-typelayer 32 of embodiment 1, an n-type layer 3 of Si doped GaN having asingle layered structure has been grown by providing, at 1050° C.,nitrogen gas, or the main carrier gas, at 9 liter/min., hydrogen gas at0.95 liter/min., a TMG carrier gas at 4 cc/min., SiH4 gas at 10 cc/min.,for a duration of 66 min.

[0054] In a light-emitting diode manufactured through the procedure ofcomparative specimen 1, the carrier concentration of the n type layerwas 1×10¹⁸ cm⁻³. When it is driven with 20 mA forward current, itemitted, like that of embodiment 1, blue-violet light of 430 nmwave-length. However, the forward operating voltage at that time was4.0V, or 0.6V higher than in embodiment 1.

[0055] (Embodiment 2)

[0056] A light emitting diode has been manufactured in the sameprocedure as in embodiment 1, except that; in the process for growing afirst n-type layer 31 of embodiment 1, a first n-type layer 31 of Sidoped GaN has been grown in the thickness of approximately 1 μm, byproviding, at 1050° C., nitrogen gas, as the main carrier gas, at 9liter/min., hydrogen gas at 0.95 liter/min., a TMG carrier gas at 4cc/min., SiH4 gas at 20 cc/min., for a duration of 30 min. After thefirst n-type layer 31 is grown and formed, the flow rate of the SiH4 gasalone was modified to 100 cc/min. to be provided by a duration of 3min., while the respective flow rate of the main carrier gas and the TMGcarrier gas have been maintained as they are, in order to grow a secondn-type layer 32 of Si doped GaN in the thickness of approximately 0.1μm.

[0057] The carrier concentration in the present exemplary embodiment 2of the respective first n-type layer 31 and the second n-type layer 32were 2×10¹⁸ cm⁻³ and 1×10¹⁹ cm⁻³. When it is driven with 20 mA forwardcurrent, it emits blue-violet light of 430 nm wavelength; and theforward operating voltage at that time was 3.3V.

[0058] As a comparative specimen 2, a light-emitting diode has beenmanufactured in the same procedure as in embodiment 2, except that; inthe process for growing a first n-type layer 31 and a second n-typelayer 32 of embodiment 2, an n-type layer of Si doped GaN having asingle layered structure has been grown by providing, at 1050° C.,nitrogen gas, as the main carrier gas, at 9 liter/min., hydrogen gas at0.95 liter/min., a TMG carrier gas at 4 cc/min., SiH₄ gas at 100cc/min., for a duration of 33 min.

[0059] In a light-emitting diode manufactured through the procedure ofcomparative specimen 2, the carrier concentration of the n-type layerwas 1×10¹⁹ cm⁻³. When it is driven with 20 mA forward current, it emits,unlike that of embodiment 2, blue-white light. In a microscopicobservation conducted to study the state of emitting the light, it hasbeen confirmed that the light was emitted only at a part of thecircumferential portion of the n-type electrode. This seems to have beencaused by cracks in the n-type layer. The forward operating voltage ofthe light-emitting diode of comparative specimen 2, when driven with 20mA forward current, was 4.8V, or 1.5V higher than in embodiment 2.

[0060] (Embodiment 3)

[0061] A light-emitting diode has been manufactured in the sameprocedure as in embodiment 1, except that; in the process for growing afirst n type layer 31 of embodiment 1, a first n-type layer 31 of Sidoped GaN has been grown in the thickness of approximately 5 μm, byproviding, at 1050° C., nitrogen gas, as the main carrier gas, at 9liter/min., hydrogen gas at 0.95 liter/min., a TMG carrier gas at 4cc/min., SiH₄ gas at 1 cc/min., for a duration of 150 min. After thefirst n-type layer 31 is grown and formed, the flow rate of the SiH₄ gasalone was modified to 20 cc/min. to be provided by a duration of 15min., while the flow rate of the respective main carrier gas and the TMGcarrier gas have been maintained as they are, in order to grow a secondn-type layer 32 of Si doped GaN in the thickness of approximately 0.5μm.

[0062] The carrier concentration in the present exemplary embodiment 3of the respective first n-type layer 31 and the second n-type layer 32were 1×10¹⁶ cm⁻³ and 2×10¹⁸ cm⁻³. When it is driven with 20 mA forwardcurrent, it emits blue-violet light of 430 nm wavelength; and theforward operating voltage at that time was 3.6V.

[0063] As a comparative specimen 3, a light-emitting diode has beenmanufactured in the same procedure as in embodiment 3, except that; inthe process for growing a first n-type layer 31 and a second n-typelayer 32 of embodiment 3, an n-type layer 3 of Si doped GaN having asingle layered structure has been grown by providing, at 1050° C.,nitrogen gas, as the main carrier gas, at 9 liter/min., hydrogen gas at0.95 liter/min., a TMG carrier gas at 4 cc/min., SiH₄ gas at 1 cc/min.,for a duration of 165 min.

[0064] In a light-emitting diode manufactured through the procedure ofcomparative specimen 3, the carrier concentration of the n-type layerwas 1×10 ¹⁷ cm⁻³. When it is driven with 20 mA forward current, itemitted, like in embodiment 3, blue-violet light of 430 nm wave-length.The forward operating voltage at that time was 4.7V, or 1.1V higher thanin embodiment 3.

[0065] (Embodiment 4)

[0066] A light-emitting diode has been manufactured in the sameprocedure as in embodiment 1, except that; in the process for growing afirst n-type layer 31 of embodiment 1, a first n-type layer 31 ofun-doped GaN has been grown in the thickness of approximately 4 μm, byproviding, at 1050° C., nitrogen gas, as the main carrier gas, at 9liter/min., hydrogen gas at 1 liter/min., a TMG carrier gas at 4cc/min., for a duration of 120 min. After the first n-type layer 31 wasgrown, a second n-type layer 32 of Si doped GaN has been grown in thethickness of approximately 0.3 μm, by providing, in succession, the maincarrier gas at 9 liter/min., hydrogen gas at 0.90 liter/min., a TMGcarrier gas at 4 cc/min., and with an addition of SiH₄ gas at 100cc/min., all of which gases are supplied for a duration of 9 min.

[0067] The carrier concentration in the present exemplary embodiment 4of the respective first n-type layer 31 and the second n-type layer 32were 2×10¹⁶ cm⁻³ and 1×10¹⁹ cm⁻³. When it is driven with 20 mA forwardcurrent, it emits blue-violet light of 430 nm wavelength; and theforward operating voltage at that time was 3.6V.

[0068] (Embodiment 5)

[0069]FIG. 3 is a sectional view showing a structure of semiconductorlight emitting device of gallium nitride compound in another embodimentof the invention.

[0070] In FIG. 3, reference numeral 100 is a substrate, which is made ofa conductive material. Preferred examples of the substrate 100 includeGaN substrate mainly composed of GaN (containing additives such as Al,In, As, P), SiC substrate mainly composed of SiC, Si substrate mainlycomposed of Si, and AlN substrate mainly composed of AlN. Further, asshown in FIG. 1, sapphire or other insulating substrate may also be usedas the substrate 100.

[0071] In particular, when a GaN substrate mainly composed of GaN isused as the substrate 100, it is preferred to form an n-layer 103 on thesubstrate directly because the n-layer 103 is also made of GaN or amaterial mainly composed of GaN. However, if the GaN substrate mainlycomposed of GaN is used as the substrate 100, either buffer layer 101 orinsertion layer 102 may be provided at the same time.

[0072] When an SiC substrate mainly composed of SiC or Si substratemainly composed of Si is used as the substrate 100, it is preferred toform a buffer layer 101 in order to enhance the crystallinity of thelayer to be laminated in the upper part. In this case, too, an insertionlayer 102 may be disposed on the buffer layer 101 so as to enhance thecrystallinity further.

[0073] When an AlN substrate mainly composed of AlN is used as thesubstrate 100, it is preferred to form an n-layer layer 103 directly onthe substrate. The reason is that the n-layer 103 is made of GaN or amaterial mainly composed of GaN, and its lattice constant is close tothat of the substrate of AlN or material mainly composed of AlN, so thata favorable crystallinity is assured without provision of buffer layer101.

[0074] The buffer layer 101 may be composed of one layer of GaN, GaAlN,AlN, AlInN, or the like, or by laminated plural layers thereof. Asmentioned above, this buffer layer 101 is preferred to be provided whenthe lattice constant of the material used as the substrate 100 isdifferent from that of the layer laminated on the upper part.

[0075] The insertion layer 102 may be composed of one layer of undopedor n-type impurity doped GaN, AlGaN, InGaN, or AlGaInN, or by laminatingplural layers thereof, and by the provision of the insertion layer 102,the crystallinity of the layer formed in the upper part is improved, ordistortion is absorbed (crack is prevented) when forming a film, so thata favorable light emitting characteristic may be obtained.

[0076] As mentioned above, depending on the material of the substrate100, both buffer layer 101 and insertion layer 102 may not be necessary,or only one of either the buffer layer 101 or the insertion layer 102may be used. When using both buffer layer 101 and insertion layer 102,it is preferred to laminate the buffer layer 101 and insertion layer 102in this sequence from the substrate 100 side.

[0077] Reference numeral 200 represents n-type layer laminated directlyon the substrate 100 or on the substrate 100 by way of at least eitherbuffer layer 101 or insertion layer 102, and the n-type layer 200 has alaminated structure forming a first n-type layer 103 of low carrierconcentration, an insertion layer 104, and a second n-type layer 105 ofhigher carrier concentration than the first n-type layer 103sequentially from the substrate 100 side. As an n-type impurity forforming the n-type layer 200, Si or Ge may be used. Generally, since aGaN material is n-type conductive in an undoped state, the n-type layer103 may be also undoped.

[0078] The insertion layer 104 is one layer selected from undoped orn-type impurity doped GaN, AlGaN, InGaN, and AlGaInN, or is formed bylaminating plural layers thereof, and the insertion layer 104 improvesthe crystallinity of the layer formed in the upper part or absorbsdistortion (prevents crack) when forming the film, so that a favorablelight emitting characteristic may be obtained. The insertion layer 104may be omitted if the first n-type layer 103 is excellent incrystallinity or distortion is small between the first n-type layer 103and second n-type layer 105, and in such a case, the first n-type layer103 and second n-type layer 105 are bonded directly.

[0079] The carrier concentration of the insertion layer 104 is notclosely related with the carrier concentration of the first n-type layer103 or carrier concentration of the second n-type layer 105. That is, asmentioned above, the insertion layer 104 is intended to improve thecrystallinity of the second n-type layer 105 or absorb distortionbetween the first n-type layer 103 and second n-type layer 105.Preferably, the carrier concentration of the insertion layer 104 may beset higher than the carrier concentration of the first n-type layer 103,so that the current flowing in the n-type layer 200 also flows smoothlyin the first n-type layer 103, and the electric resistance of the n-typelayer 200 may be lowered and the efficiency may be enhanced. Further,preferably, the carrier concentration of the insertion layer 104 shouldbe lower than the carrier concentration of the second n-type layer 105.

[0080] Reference numeral 107 is a luminous layer, which is formeddirectly on the second n-type layer 105 or laminated by way of aninsertion layer 106.

[0081] The luminous layer 107 composed of InGaN or other material may bedoped with zinc or Si or the like, and a luminous layer making use ofimpurity level may also be formed, or it may be also formed as anundoped luminous layer of film thickness of 10 nm or less by making useof quantum level. Specifically, it may be formed as a single quantumwell structure composed of one layer of InGaN, or as a multiple quantumwell structure composed by laminating at least two layers of InGaN layerand GaN layer alternately, or laminating at least two layers of InGaNlayer different in concentration of In alternately. Herein, an undopedcomposition refers to a film formed without adding p-type impurity orn-type impurity. If the luminous layer 107 is the multiple quantum wellstructure, the undoped luminous layer means the well layer is undoped,and it does not matter whether the barrier layer is doped or not.

[0082] The insertion layer 106 is formed by using at least one layerselected from undoped or n-type impurity doped GaN, AlGaN, InGaN, andAlGaInN. By using one material only selected from the group, one layeror plural layers may be laminated, or by laminating plural materialsselected from the group, at least one layer may be composed of adifferent material from the other layer.

[0083] By disposing the insertion layer 106, the crystallinity of thelayer formed in the upper part may be improved, the diffusionconcentration of n-type impurity into the luminous layer 107 may be keptwithin an allowable range, or distortion can be absorbed (crack can beformed) when forming a film, so that a favorable light emittingcharacteristic may be obtained. The insertion layer 106 may be omittedif the second n-type layer 105 is excellent in crystallinity, the degreeof diffusion of n-type impurity into the luminous layer 107 is small, ordistortion is small between the luminous layer 107 and second n-typelayer 105, and in such a case, the second n-type layer 105 and luminouslayer 107 are bonded directly.

[0084] Reference numeral 109 is a p-type clad layer, which is directlybonded on the luminous layer 107, or disposed by way of an insertionlayer 108.

[0085] As the p-type clad layer 109, one layer selected from AlGaN, GaN,and AlGaInN may be used, or plural layers selected from the group may belaminated. As a p-type contact layer 110 laminated on the p-type cladlayer 109, GaN, InGaN, or AlGaN may be used. As the p-type impurity tobe doped in the p-type clad layer 109 and p-type contact layer 110, Mgor the like may be used.

[0086] The insertion layer 108 is formed by using at least one layerselected from undoped or p-type impurity doped GaN, AlGaN, InGaN, andAlGaInN. That is, by using one material only selected from the group,one layer or plural layers may be laminated, or by laminating pluralmaterials selected from the group, at least one layer may be composed ofa different material from the other layer.

[0087] The insertion layer 108 is intended to prevent diffusion ofp-type impurity into the luminous layer 107 or to enhance the holemobility in the insertion layer 108. This insertion layer 108 may beomitted if prevention of diffusion of p-type impurity into the luminouslayer 107 can be realized by other means, the diffusion concentration ofp-type impurity into the luminous layer 107 is in an allowable range, orthe hole mobility in the p-type clad layer 109 is high.

[0088] On the p-type contact layer 110, a p-side electrode 111 isformed, while an n-side electrode 112 is disposed on the second n-typelayer 105 of the n-type layer 200. The material for forming the n-sideelectrode 112 is aluminum (Al), titanium (Ti), or other metal. In thisembodiment, the p-type layer is composed of a two-layer structure ofp-type clad layer 109 and p-type contact layer 110, but it may also becomposed in a single-layer structure or a multi-layer structure of threelayers or more.

[0089] Herein, in manufacture of semiconductor light emitting device ofgallium nitride compound, it has been already explained that thefrequency of occurrence of cracks when growing the film becomes high,thereby lowering the manufacturing yield if the dopant of the n-typeimpurity in the n-type layer is increased or the thickness of the n-typelayer is increased to reduce the operating voltage of the device. Suchoccurrence of cracks seem to be caused by distortion formed in then-type layer grown by doping the n-type impurity at high concentrationand increase of distortion amount by increasing the layer thickness.

[0090] It was a conventional technology that the decrease of operatingvoltage by optimizing the n-type impurity dopant amount and the layerthickness of the n-type layer is limited because of existence oftrade-off about occurrence of cracks between the n-type impurity dopantamount and the layer thickness of the n-type layer. In other words,since trade-off about occurrence of crack in n-type layer exists betweenthe n-type impurity dopant amount and the layer thickness of the n-typelayer, decrease of operating voltage is limited if they are optimized.

[0091] By contrast, in this embodiment, the n-type layer 200 has alaminated structure including the first n-type layer 103 of low carrierconcentration and second n-type layer 105 of higher carrierconcentration sequentially from the substrate 100 side, and by themulti-layer structure of the n-type layer 200 disposing the n-sideelectrode 112 on the second n-type layer 105, the operating voltage canbe lowered and the manufacturing yield can be maintained at high level.This is explained below.

[0092] In the semiconductor light emitting device of gallium nitridecompound of the embodiment shown in FIG. 3, the n-type layer 200 has alaminated structure including the first n-type layer 103 of low carrierconcentration and second n-type layer 105 of higher carrierconcentration sequentially from the substrate 100 side. That is, afterforming the first n-type layer 103 of low carrier concentration at thesubstrate 100 side, the second n-type layer 105 of higher carrierconcentration is formed on the first n-type layer 103 either directly orby way of the insertion layer 104, and the n-side electrode 112 isformed on the second n-type layer 105.

[0093] By such laminated structure forming the first n-type layer 103and second n-type layer 105 different in the carrier concentrationsequentially from the substrate 100 side, if the carrier concentrationis lowered by decreasing the n-type impurity dopant amount of the firstn-type layer 103, and first n-type layer 103 can be formed as a thickfilm, and increase of resistance and occurrence of crack in the firstn-type layer 103 can be suppressed at the same time. On the first n-typelayer 103, by forming the n-side electrode 112 on the second n-typelayer 105 heightened in the carrier concentration by increasing then-type impurity dopant amount larger than the first n-type layer 103,the contact resistance between the second n-type layer 105 and n-sideelectrode 112 can be decreased, and thereby the operating voltage of thelight emitting device can be lowered and the power consumption can besaved.

[0094] Thus, difference in the carrier concentration between the firstand second n-type layers 103, 105 can encourage optimization in bothaspects of decrease of operating voltage and prevention of occurrence ofcracks. Specific numerical definitions of the carrier concentrations areas follows according to the knowledge of the present inventors.

[0095] The carrier concentration of the first n-type layer 103 ispreferred to be in a range of 1×10¹⁶ cm⁻³ to 2×10¹⁸ cm⁻³. As far as thiscarrier concentration range is satisfied, the n-type layer 103 may beundoped. When the carrier concentration of this first n-type layer 103is smaller than 1×10¹⁶ cm⁻³, the series resistance by the first n-typelayer 103 itself increases, and the operating voltage of the devicetends to be higher, or when the carrier concentration is larger than2×10¹⁸ cm⁻³, cracks are likely to occur.

[0096] The carrier concentration of the second n-type layer 105 higherthan the carrier concentration of the first n-type layer 103 ispreferred to be 2×10¹⁸ cm⁻³ to 1×10¹⁹ cm⁻³. When the carrierconcentration of this second n-type layer 105 is smaller than 2×10¹⁸cm⁻³, it is hard to decrease the contact resistance to the n-sideelectrode 8 sufficiently, or when larger than 1×10¹⁹ cm⁻³, thecrystallinity of the layer tends to be worse, and the crystallinity ofthe luminous layer or p-type layer grown thereon becomes poor, and thelight emitting output may decline.

[0097] The film thickness of the second n-type layer 105 should besmaller than that of the first n-type layer 103, preferably in a rangeof 0.1 to 0.5 μm. If thinner than 0.1 μm, it is hard to control thedepth of etching for exposing the surface of the n-type layer 3 byremoving part of the p-type layer and luminous layer 107 by the p-typeclad layer 109 and p-type contact layer 110. If thicker than 0.5 μm, thecrystallinity of the second n-type layer 105 is poor, and thecrystallinity of the luminous layer 107, p-type clad layer 109 andp-type contact layer 110 grown on this second n-type layer 105 isimpaired, and the light emitting output may be lowered.

[0098] The film thickness of the first n-type layer 103 is preferred tobe in a range of 1 to 5 μm. If thinner than 1 μm, the series resistanceof the device increases and the operating voltage tends to be higher, orif thicker than 5 μm, cracks are likely to occur.

[0099] One advantage of the present invention is that the contactresistance between the n-type layer and the n-side electrode formedthereon is lowered by providing an n-type layer stacked on a substratein a two-layered structure composed of a first n-type layer of smallercarrier concentration and a second n-type layer of greater carrierconcentration. As a result, the operating voltage of a light-emittingdevice is lowered and the power consumption reduced.

[0100] Another advantage is that the occurrence of cracks in alight-emitting device is suppressed through a specified carrierconcentration of the respective first and second n-type layers, inaddition to reduction in the power consumption.

[0101] In a variation of the foregoing, the occurrence of cracks in alight-emitting device can be suppressed through a specified relationshipin the layer thickness between the first and the second n-type layers.

[0102] In yet another variation, the occurrence of cracks can beeffectively suppressed through a specified range of respective layerthickness values in the first and the second n-type layers. In addition,the depth of etching, when partially etching the p-type layer and thelight-emitting layer off for having the surface of n-type layer exposed,can be maintained at a level of high precision.

[0103] Of course, it should be understood that a wide range ofmodifications can be made to the exemplary embodiments described above.It is therefore intended that the foregoing detailed description beregarded as illustrative rather than limiting and that it be understoodthat it is the following claims, including all equivalents, which areintended to define the scope of this invention.

What is claimed is:
 1. A semiconductor light emitting device of galliumnitride compound comprising: a substrate, an n-type component composedof GaN or a material mainly made of GaN, disposed on said substratedirectly or by way of other layer, an undoped luminous part disposeddirectly or by way of other layer at the opposite side of the substrateside in said n-type component, a p-type component disposed directly orby way of other layer at the opposite side of the n-type component sidein said luminous part, and electrodes disposed in said p-type componentand n-type component individually, wherein said n-type component has afirst region and a second region in a thickness direction, said firstregion and second region are disposed sequentially from the substrateside, the second region and substrate do not contact with each other,the first region and second region are disposed directly or by way ofother layer, the second region is set at a higher carrier concentrationthan the first region, and its carrier concentration is 2×10¹⁸ cm⁻³ to1×10 ¹⁹ cm⁻³.
 2. The semiconductor light emitting device of galliumnitride compound of claim 1, wherein the substrate is conductive.
 3. Thesemiconductor light emitting device of gallium nitride compound of claim2, wherein the substrate is a GaN substrate mainly composed of GaN (withadditive material such as Al, In, As, P), an SiC substrate mainlycomposed of SiC, an Si substrate mainly composed of Si or an AlNsubstrate mainly composed of AlN.
 4. The semiconductor light emittingdevice of gallium nitride compound of claim 1, wherein an insertionlayer composed of one layer of undoped or n-type impurity doped GaN,AlGaN, InGaN, or AlGaInN, or by laminating plural layers thereof isdisposed between the first region and second region of the n-typecomponent.
 5. The semiconductor light emitting device of gallium nitridecompound of claim 1, wherein an insertion layer composed of one layer ofundoped or n-type impurity doped GaN, AlGaN, InGaN, or AlGaInN, or bylaminating plural layers thereof is disposed between the n-typecomponent and luminous part.
 6. The semiconductor light emitting deviceof gallium nitride compound of claim 5, wherein said insertion layer isa laminated layer of plural films, and at least one of the films is madeof a material different from other material.
 7. The semiconductor lightemitting device of gallium nitride compound of claim 6, wherein saidinsertion layer is formed by alternately laminating layers of differentmaterials.
 8. The semiconductor light emitting device of gallium nitridecompound of claim 1, wherein an insertion layer composed of one layer ofundoped or p-type impurity doped GaN, AlGaN, InGaN, or AlGaInN, or bylaminating plural layers thereof is disposed between the luminous partand p-type component.
 9. The semiconductor light emitting device ofgallium nitride compound of claim 8, wherein said insertion layer is alaminated layer of plural films, and at least one of the films is madeof a material different from other material.
 10. The semiconductor lightemitting device of gallium nitride compound of claim 9, wherein saidinsertion layer is formed by alternately laminating layers of differentmaterials.